This invention relates to fastener driving devices, and more particularly, to portable pneumatically powered actuated fastener driving devices.
Pressure operated fastener driving devices are well-known and typically include a portable housing defining a guide track, a magazine assembly for feeding successive fasteners laterally into the guide track, a fastener driving element slidable in the drive track, a piston and cylinder unit for moving the fastener driving element through a cycle which includes a drive stroke and a return stroke, and a main valve assembly for controlling the communication of the cylinder with air under pressure communicated with the device and with the atmosphere to effect cycling, and a manually operable valve for controlling the main valve assembly through pilot pressure. These devices typically include a handle defining a pressure reservoir therein, communicating with line pressure generally at 90 to 100 psig.
In certain circumstances, for example when operating the fastener driving tool at maximum energy with respect to a workpiece such as soft wood, only a fraction of the energy is required to drive the fastener into the workpiece. Thus, the tool must absorb the excess energy which significantly reduces tool life. It can easily be appreciated by those skilled in the art that if maximum energy is used to drive the fastener into a soft workpiece, the fastener driving tool may drive the fastener too deeply into the workpiece. Thus, there exists a need to reduce the amount of excess energy absorbed by the tool and also to control the energy imparted to the fastener. When using conventional fastener driving tools, tool energy is normally regulated by changing the line pressure. For example, in certain fastener driving tools, at 100 psig line pressure, tool energy is, for example, approximately 162 in-lbs. In certain circumstances, line pressure can be reduced so as to operate the tool at a reduced energy and thus, lengthen tool life. For example, the line pressure may be reduced to 70 psig or less, which reduces tool energy to, for example, 117 in-lbs, requiring less energy to be absorbed by the tool.
In certain circumstances, tool energy can be reduced by employing a fixed orifice in an air flow path between the reservoir and the cylinder to restrict air flow from the reservoir creating a pressure drop over the piston at the cylinder. The pressure drop during tool actuation reduces the tool energy. Thus, if the above-mentioned conventional tool utilizes a fixed orifice, standard tool energy may be reduced from 162 in-lbs to approximately 117 in-lbs while maintaining 100 psig line pressure. If further reduction of tool energy is required, the line pressure may be reduced to a satisfactory level.
At a typical field location, line pressure is generally constant and set at a maximum value. Generally, there is no convenient way to regulate the line pressure, therefore, the tool energy cannot be reduced if desired to prolong the tool life. Accordingly, a need exists to regulate the tool energy of a portable pneumatic fastener driving device without adjusting the line pressure.